Demethanization of refinery gases



*May 16, 1967 l F. rSAAcsoN 3,319,428

DEMETHANIZATION OF REFINERY GASES May 16, 1967 F. ISAACSON I 3,319,428

DEMETHANIZATION OF REFNERY GASES Filed Jan. 16, 1964 2 sheets-sheet 2 157' .STAGE 2ND STAGE 31?@ STAGE 4TH .S7-AGE 3,319,428 DEMETHANIZATION OF REFINERY GASES Franklyn Isaacson, Springfield, NJ., assigner to Foster Wheeler Corporation, New York, NX., a corporation of New York Filed lan. 16, 1964, Ser. No. 338,055 4 Claims. (Cl. 62-28) This invention relates to a low temperature fractionation process, and in particular to a novel arrangement for low temperature fractionation in the separation of methane and lighter materials from ethylene and heavier materials, in the production of ethylene.

In the recovery of ethylene from sources such as pyrolysis gas effluent and refinery gases, the separation of Vmethane and lighter materials from the gas stream, known as demethanization, is the most costly step of several in the over-all process, and considerable effort is being expended in the field to reduce the cost.

Several methods have been proposed for demethanizing or separating ethylene from its source. One such method involves fractionation of the components, and is characterized by a low process temperature and a high pressure in the range of four hundred (400) to six hundred (600) p.s.i.a. The present invention is concerned with this method.

Generally for conventional fractionation units operating in this pressure range, demethanization is accomplished in a single distillation column. Since the critical temperature for ethylene is 49 F., refrigeration is necessary to condense ethylene reflux for the column. A great deal of refrigeration is generally required, and the head temperature for the distillation column usually will be in the range of -140 to 250 F. Design problems for straight fractionation apparatus are centered around these low temperature process requirements, and the refrigeration system represents a significant portion -of the cost of an ethylene unit.

In accordance with the present invention, it is proposed to reduce the low-temperature refrigeration duty by running 'the demethanizer at a higher pressure. This raises the temperature level required to co-ndense reflux for any particular tail gas composition. However, a limit is imposed on the pressure increase by the composition of the tower bottoms. In many cases, the tower bottoms ternperature approaches the critical temperature as the pressure in the tower exceeds 600 p.s.i.a.

Therefore, by the invention, it is further proposed to divide the demethanizer tower into a stripping section and a rectification section, wherein the stripper is maintained at a pressure lower than the pressure in the rectification section. With the lower stripping pressure, the approach of bottoms temperature to critical temperature is no longer a problem, and the pressure in the rectification section may be raised to any desired level to reduce refrigeration load.

Also, in accordance with the invention, there is provided a method for the separation of components of a gaseous feed mixture into a primarily ethylene fraction and into a lower boiling normally gaseous fraction containing methane and hydrogen. This method includes the steps of compressing the feed mixture to a superatmospheric press-ure, preferably in the range of from 400 to 800 p.s.i.a., and cooling it to a temperature, which may be in the range of to -80 F. respectively, at or near the boiling point of the mixture but above the boiling point under this pressure. The mixture is introduced at this pressure and temperature into a Separation Zone of a fractionation tower, and the lower boiling cornponents including methane and hydrogen are withdrawn as a portion of the overhead product. The remainder of the overhead product is cooled, condensed and refluxed United States Patent O 3,319,428 Patented May 16, 1967 ice into the separation zone and passed in countercurrent contact with the feed mixture.

The condensate is withdrawn from the bottom of the separation zone and expanded through a pressure reducing valve into a stripping zone, maintained at a substantially lower pressure, in the range of or near 250 p.s.i.a, and at a higher temperature at or near -10 F. At the bottom of the stripping zone, a portion of the condensate is reboiled, the vapor passing in countercurrent contact with the condensate through the stripping zone. The remainder of the condensate is removed as bottoms product from the zone, and the vapor product from the zone is withdrawn and reintroduced into the system at a stage of compression of the feed mixture.

In a preferred arrangement of the invention, the separation zone and the stripping zone are part of a single fractionation tower, the stripping zone being enlarged over that of the separation Zone to accommodate for the increased volume of the expanded condensate.

In the embodiment of the invention, the condensate is passed through a series of flash drums between the separation zone and the stripping zone. The vapors from the iiash drums are reintroduced at stages of compression of the feed mixture, and the remainder of the condensate is passed into the stripping zone for separation into ethylene and an overhead product. In this embodiment, also, the stripper overhead product is reintroduced into the system at a stage of compression of the feed mixture.

The invention and advantages thereof will become more apparent on consideration of the following description with reference to the accompanying drawings, in which:

FIGURE l represents in part a schematic flow diagram of a process for the recory of ethylene in accordance with the invention; and

FIGURE 2 illustrates a modification thereof.

Referring to FIG. l, in an ethylene recovery process of the type to which the invention relates, pyrolysis furnace gases, which have been quenched and compressed in vessel 12 and compressor system 14 are subjected to primary separation and fractionation in separator 16 and4 tower 18 for removal of C3 'and heavier hydrocarbons. The gaseous product, in line 20, is then compressed and cooled by stages in compressors 22 through 28 and acetylene, if desired, is removed in system 30, schematically indicated.

The gaseous fraction, consisting largely of ethane and lighter hydrocarbons, including methane and hydrogen, is then subjected to cooling in exchanger 32 where it is cooled to a temperature of 50 to 60 F. This results in condensation of most of the water vapor in the feed, but the moisture content may be further reduced to a minimum level in a drier 34. Additional cooling of the feed gas is then -obtained in heat exchanger 36 lowering the temperature to a range of from 10 to 80 F., and the compressed and cooled gas is fed to a demethanizer 38 where it is fractionated under pressure.

If desired, extraneous refinery gases may be a-dded to the feed gas in line 40 following the primary fractionation step. Other variations can be made in the system. For instance, changes in the sequence of steps may be made readily, depending on many factors including product requirements.

The demethanizer 38 is divided into two sections, a rectification section 42 into which the gas is fed, and a stripping section 44. As illustrated in FIGURE l, the` p.s.i.a., the bottoms temperature would have been about 50 F., and at 600 p.s.i.a., about 100 F., or above the critical temperature for a typical bottoms composition. In accordance with the invention, and at the lower pressure, the bottoms temperature may be about l0 F., again epending on the design of the demethanizer and overhead sy'stem used, but well below the bottoms critical temperature.

The column of the stripping section is increased over that of the rectification section to accommodate the fio'w 'at a reduced pressure level.

ne advantage of the invention should now be apparent. The present invention in providing a means for maintaining the stripping pressure lower than the pressure in the rectification section, eliminates the problem of approach of bottoms temperature to critical temperature as the pressure is increased. lt follows that the rectification section pressure can be raised to reduce the required refrigeration load, thereby realizing an economy in reduced refrigeration costs` The rectification section 42 of the demethanizer tower is a conventional bubble-cap or sieve tray column, and is suitably insulated and structurally reinforced for a very low temperature environment. The vapor feed mixture passes into the tower and through it in countercurrent flow with liquid reflux, and passes into an overhead exchange system 48 wherein further refrigeration is effected on the mixture. Evolving from the refrigerated mixture are methane and lighter components, a portion of which comprises tail gas in li'ne 50 and a portion of which is recycled in line 52 int-o the system at a point S4 prior to the first stage compression, or at some other appropriate stage of compression. The remaining liquid is refluxed in line 56 back into the rectification section of the tower and passes in countercurrent Contact with the feed mixlle;

Leading from the upper rectification section above the separating disc 46 and into the stripping section 44 is a line 58 for conveying liquid condensate, the latter being at a high pressure. A reducing valve 60 is disposed in the line 578 so that the stripping section may be maintained at a lower pressure.

The reflux passes to the bottom of the stripping `section Where the bulk of it is vaporized in reboiler 62 heated by coil 6,4. The vaporized portion of the reux ows upwardly against the liquid flow and exits in line 65 at the top of the section and immediately below the separating disc 46, and since the vapor is at relatively low pressure, it can be introduced or recycled into the system at point 68 prior lto the 4th stage of compression. The particular point at which the stripper vapor is introduced into the system depends on process design factors. The bottoms mixture, consisting principally of ethane and ethylene, and substantially free of methane and hydrogen, is fed through line 70 for further processing.

Preferably the rectification section is maintained at a pressure of about 600 p.s.i.a. or higher thereby lowering the refrigeration dutyin the overhead exchange system. For instance, depending on the particular reflux or overhead system, the temperature level for the overhead product may be raised from 145 F. at 500 p.s.i.a. to 150 F. at 600 p.s.i.a.

Further, in the system illustrated, heat `for the reboiler may be supplied by condensing refrigerant. But in addition, at sufficiently low stripper pressure, bottoms temperature may be low enough to permit using the reboiler as the equivalent of a refrigeration sour-ce7 thereby reducing original equipment costs and effecting other savings. In this respect, the heat required for reboiling may be obtained from refrigeration of the feed, or alternatively, of the reflux, eliminating in equipment a heat exchanger and piping but additionally effecting a more eflicient use of process heat.

The specific pressure, temperatures, flow rates, compression rates, tray and reboil requirements, and reflux requirements will be interrelated and based on the nature of the need, and requirements of operation.

FIG. 2 illustrates a modified form of the invention. ln this arrangement, a gas stream from a suitable source is fed directly into the demerthanization tower rectification section 72, after being suitably refrigerated and dried. The feed passes upwardly through the tower in countercurrent contact with the tower reflux, the reflux owing downwardly and from the rectification section through reducing valves 74a, b, and c into a series of flash drums, 76a, b, and c. Each flash drum is in contact with an intermediate stage of a compressor 78. In the ash drums, the methane and lighter overhead is flashed to the compresser intermediate stage, and the heavier net product flows to the stripping section, through line 82, where it is subjected to reboiling. The vapor from the stripping section is fed in line 84 to a final stage of compression. This system is suitable where a large pressure difference exists between the rectification section and the stripping section.

Although the invention has been described lwith reference to specific embodiments, many variations within the spirit and scope of the invention, as defined in the following claims, will be apparent to those skilled in the art. For instance, in the examples described and illustrated, not all of the heat exchangers required are shown or discussed. The source or sources of feed gas and the exact point at which any source may be added to the system may be varied. The number of stages of feed gas compression, the methods of gas preparation, the heat exchange arrangement, and the overhead exchanger -system may also be varied.

What is claimed is:

l. A method for the separation of components of a gaseous feed mixture containing a methane and lighter fraction and an ethylene and heavier fraction, comprising the steps of compressing the mixture to a superatmospheric pressure in the range of 600 to 800 p.s.i.a. and cooling the mixture to a temperature in the range of 10 to F., respectively, above but near the boiling point of the mixture under this pressure; introducing the feed mixture at this pressure and temi perature into a separation Zone near the bottom of said zone; and maintaining the bottom of the separation zone at approximately the pressure and temperature of the feed mixture; removing from the separation zone an overhead product of lower boiling components including methane and hydrogen;

cooling and condensing a portion of the overhead product and refluxing the condensate to the separation Zone for countercurrent contact with vapor in the separation Zone; withdrawing liquid bottoms from the base of the sep-aration zone at approximately the temperature and pressure of the feed mixture and expanding the liquid bottoms through pressure reducing means to a pressure of about 250 p.s.i.a. and into a stripping zone;

reboiling a major portion of the condensate in said stripping zone maintaining in the bottom of the stripping Zone a temperature well below stripping zone bottoms critical temperature;

removing the remainder of the condensate as a vbottoms product from said stripping zone;

and withdrawing a vapor product from the top of the stripping zone and reintroducing it into the system during an initial compression stage for the feed mixture.

2. A method according to claim 1 wherein the temperature in the bottom of the stripping zone is about 10 F.

3. A method for the separation of components of a gaseous feed mixture containing a methane and lighter fraction and an ethylene and heavier fraction, comprising the steps of compressing the mixture in at least three stages of compression to a superatmospheric pressure in the range of 600 to 800 p.s.i.a. and cooling the mixture to a temperature in the range of to 80 F., respectively, above but near the boiling point of the mixture under this pressure; introducing the feed mixture at this pressure and temperature into a separation zone near the bottom of said zone, and maintaining the bottom of the separation zone at approximately the pressure and temperature of the feed mixture; removing from the separation zone an overhead product of lower boiling components including methane and hydrogen; reintroducing a portion of the overhead product into the feed mixture prior to the rst stage of compression; cooling and condensing the remainder of the overhead product and reuxing the condensate to said separation zone and passing it in countercurrent contact with the vapor in the separation zone; withdrawing liquid bottoms from the base of the separation zone at approximately the pressure and temperature of the feed mixture and expanding the liquid bottoms through at least one pressure reducing valve means to a pressure of about 250 p.s.i.a. and into a stripping zone; reboiling a major portion of the condensate in said stripping zone maintaining in the bottom of the stripping zone a temperature well below stripping zone bottoms critical temperature; removing the remainder of the condensate as a bottoms product from said zone; and withdrawing a vapor product from the top of said stripping zone and mixing it with said gaseous feed mixture subsequent to the third stage of compression of the feed mixture. 4. A method for the sepa-ration of components of a gaseous feed mixture comprising the steps of compressing the mixture in at least three stages of compression to a superatmospheric pressure and cooling it to a temperature near its boiling point under this pressure;

introducing the mixture at this pressure and temperature into a fractionation zone and lremoving an overhead product including lower boiling components;

reintroducing the lower boiling components into the feed mixture prior to the first stage of compression;

cooling and condensing the remainder of the overhead product and reflexing it to said fractionation zone;

withdrawing the condensate from the base of the fractionation zone and expanding it through a series of pressure reducing valves into a series of flash drums;

removing the vapor products from each of said Hash drums and reintroducing said products into the feed mixture at one or more stages of compression of the gaseous feed mixture;

introducing the remainder of the condensate into a stripping Zone;

reboiling a portion of said condensate in said stripping zone and removing the remainder of the condensate as a bottoms product from said zone;

and withdrawing a vapor product from said stripping zone and mixing it with said gaseous feed mixture subsequent to the third stage of compression of the feed mixture.

References Cited by the Examiner UNITED STATES PATENTS 1,853,743 4/ 1932 Pollitzer 62-24 2,500,353 3/1950 Gantt 62-28 X 2,573,341 10/1951 Kniel 62-28 X 2,777,305 1/1957 Davison 62-28 X 2,804,488 8/ 1957 Cobb 62-23 2,973,834 3/ 1961 Cicalese.

3,160,489 12/1964 Brocoff et al 62-26 X 3,260,058 7/1966 Ray et al 62--26 X NORMAN YUDKOFF, Primary Examiner. V. W. PRETKA; Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,319 ,428 May 16 1967 Franklyn Isaacson It s hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below Column 2, line 33, for "recory" read recovery column 3, line 9, for "column" read Volume line 6l, for "-l50" read -l30 column 4 line 5 for "demerthanzation" read demethanzaton column 6, line 10, for "reflexing" read refluxng Signed and sealed this 24th day of September 1968.

(SEAL) Attest:

EDWARD I. BRENNER Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer 

1. A METHOD FOR THE SEPARATION OF COMPONENTS OF A GASEOUS FEED MIXTURE CONTAINING A METHANE AND LIGHTER FRACTION AND AN ETHYLENE AND HEAVIER FRACTION, COMPRISING THE STEPS OF COMPRESSING THE MIXTURE TO A SUPERATMOSPHERIC PRESSURE IN THE RANGE OF 600 TO 800 P.S.I.A. AND COOLING THE MIXTURE TO A TEMPERATURE IN THE RANGE OF -10* TO -80*F., RESPECTIVELY, ABOVE BUT NEAR THE BOILING POINT OF THE MIXTURE UNDER THIS PRESSURE; INTRODUCING THE FEED MIXTURE AT THIS PRESSURE AND TEMPERATURE INTO A SEPARATING ZONE NEAR THE BOTTOM OF SAID ZONE; AND MAINTAINING THE BOTTOM OF THE SEPARATION ZONE AT APPROXIMATELY THE PRESSURE AND TEMPERATURE OF THE FEED MIXTURE; REMOVING FROM THE SEPARATION ZONE AN OVERHEAD PRODUCT OF LOWER BOILING COMPONENTS INCLUDING METHANE AND HYDROGEN; COOLING AND CONDENSING A PORTION OF THE OVERHEAD PRODUCT AND REFLUXING THE CONDENSATE TO THE SEPARATION ZONE FOR COUNTERCURRENT CONTACT WITH VAPOR IN THE SEPARATION ZONE; WITHDRAWING LIQUID BOTTOMS FROM THE BASE OF THE SEPARATION ZONE AT APPROXIMATELY THE TEMPERATURE AND PRESSURE OF THE FEED MIXTURE AND EXPANDING THE LIQUID BOTTOMS THROUGH PRESSURE REDUCING MEANS TO A PRESSURE OF ABOUT 250 P.S.I.A. AND INTO A STRIPPING ZONE; REBOILING A MAJOR PORTION OF THE CONDENSATE IN SAID STRIPPING ZONE MAINTAINING IN THE BOTTOM OF THE STRIPPING ZONE A TEMPERATURE WELL BELOW STRIPPING ZONE BOTTOMS CRITICAL TEMPERATURE; REMOVING THE REMAINDER OF THE CONDENSATE AS A BOTTOMS PRODUCT FROM SAID STRIPPING ZONE; AND WITHDRAWING A VAPOR PRODUCT FROM THE TOP OF THE STRIPPING ZONE AND REINTRODUCING IT INTO THE SYSTEM DURING AN INITIAL COMPRESSION STAGE FOR THE FEED MIXTURE. 